The present invention relates to a method for the solidification, in a manner which protects the environment against contamination, of waste materials obtained during reprocessing of irradiated nuclear fuel and/or breeder materials in a matrix of a borosilicate glass type, in which highly radioactive solutions or slurries containing the waste materials in dissolved or suspended form are evaporated in a vessel in the presence of glass former substances until they are dry, the dry residue is calcinated and the calcinate is melted together with the glass formers while the waste gases are discharged to the environment.
In order to solidify highly radioactive waste materials obtained during reprocessing of irradiated nuclear fuels, it has been known for a long time to use masses of glass or a glass-like material, for example, masses of the type of a borosilicate glass or of the type of a phosphate glass. A series of articles report on experiments to evaporate aqueous radioactive solutions or slurries until they are dry, to calcinate the dry residue and to incorporate the calcinate in such matrices through melting with the addition of glass formers.
In Great Britain, for example, the FINGAL process has been developed in which the waste solution and the glass forming additives are supplied and pretreated in separate systems, and are mixed together only shortly before entrance into the process vessel in which the glass mass is produced by melting. See, J. R. Grover, W. H. Hardwick, R. Gayler, M. H. Delve: Report of the United Kingdom Atomic Energy Authority, Research Group, Nr. AERE-R-5188, 1966. The process vessel is inserted into a high temperature furnace which is divided into a plurality of separate heating zones. Two further vessels are connected with the process vessel in series, the further vessels being provided with a primary or secondary filter, respectively, for cleaning the waste gas. In order to prevent condensate formation in the vessels containing the filters, the two filter containing vessels are inserted in furnaces. These filters are provided to retain suspended matter and volatile fission products and are melted into the glass matrix when the filters become fully charged. The further components of the waste gas system are a condenser, a nitric oxide absorber in which nitric acid is recovered, a base liquor washer and an absolute filter.
For the FINGAL process, the solidification matrix can be a phosphate glass or a borosilicate glass. Incorporation of the waste material in borosilicate glass is preferred because the highly corrosive phosphate glass melt led to considerable difficulties in spite of certain good properties, such as, for example, low melting temperature and relatively good dosability of the glass formers. Due to the required limitation of the operating temperature of the glass melt to about 1100.degree. C. in order to assure sufficient lifetime of the system components, the use of a borosilicate glass generally does not permit the incorporation of more than about 30 percent by weight waste materials or waste oxides, respectively, in the final product.
The FINGAL process itself has been described as follows: In a stirring vessel, a pumpable suspension was produced of finely ground borax, silicon dioxide and nitric acid. It has a relatively low tendency to settle. The waste solution, which was received from the reprocessing system in a pre-concentrated form, is pretreated in an additional vessel, i.e. is brought to the chemical composition required for solidification. Then, waste solution and glass formers are separately pumped into the process vessel and are mixed together shortly before they enter it. The start of the introduction of the mixture takes place at relatively low temperatures. In the process vessel, layers must form in which the following individual process steps can take place:
(1) a first layer in which there is evaporation of the water and the nitric acid, and removal of the resulting nitric oxides; PA1 (2) a second layer in which there is calcination and possibly sintering; and PA1 (3) a third layer in which there is melting. PA1 heating of the process vessel to operating temperature: about 6 hours; PA1 melting away deposits at the head of the process vessel: about 3 hours; PA1 evaporating, calcining, and melting until the melt fills 70% of the vessel volume: about 24 to 30 hours; PA1 cooling and normalizing period: about 20 hours. PA1 (1) denitration of a preconcentrated fission product solution with the addition of formaldehyde in a modified evaporator; PA1 (2) mixing the denitrated solution with the glass formers; PA1 (3) drying the suspension on a roller dryer; PA1 (4) vitrification of the dry residue in an induction heated crucible; PA1 (5) purification of the waste gas with recovery of concentrated nitric acid.
The pretreatment of the waste solution in an additional vessel comprises a further concentration of the pre-concentrated waste solution together with careful control of chemical composition of the mixture, including glass former additives. If necessary, additional glass former components can be added into this vessel. This pretreatment is required not only for the FINGAL process but also for the RLG process, for the continuous pot glass process, and for the Piver process, which were described in the following.
For this reason, separate heating zones must be provided in the process vessel. The lower portion of the process vessel is heated to about 1050.degree. C. in order to melt the calcinate only after the calcinate layer has become thick enough so that no waste solution can flow through and under the calcinate which would interfere with the normal process sequence. With increasing quantities introduced and increasing masses of the glass melt, layers (1) and (2) travel upwardly. The heating energy for the separate heating zones of the high temperature furnace is selected correspondingly. When the process vessel has been filled with glass melt to about 70% of its volume, the introduction of waste solution and glass formers is terminated. The feeder line is then rinsed clean with water and the temperature of the heater in the region of the head of the vessel is increased in order to melt deposits which may have accumulated there. Then, the high temperature furnace is switched off, the process vessel is cooled with air and uncoupled from the supply lines, and removal from the high temperature melting furnace and sealed. The sealed process vessel serves as a storage vessel and can be transported to a storage location. The process is thus discontinuous, i.e. material is fed in only until the glass melt has reached 70% of the process vessel volume. Then, the first filter vessel which was immediately below the process vessel and which now contains a charged filter is introduced into the high temperature melting furnace and now serves as the new process vessel. Before the start of renewed introduction of waste solution and glass formers into the new process vessel, the vessel is heated to about 420.degree. C. causing a solder connection, with which the filter has connected to the waste gas line, to be melted and the filter to drop to the bottom of the vessel. There, the filter will be enclosed in glass during the further process sequence.
In a process vessel of 1500 m in length and a diameter of 150 mm, an intake rate of about 4.5 liter per hour is supposed to be attainable in the FINGAL process. The times required for the process are supposed to be as follows:
A similar process is the rising-level-glass process (RLG) developed in the USA in which, as in the FINGAL process, the glass melt mass containing the waste substances increases in the course of the process and the individual process steps of (1) evaporating and drying, (2) calcinating, and (3) melting, take place simultaneously in delimited zones. When the aqueous phase has reached a certain level or layer thickness, respectively, in the process vessel the amount of waste solution introduced is reduced and is adapted to the existing evaporation output. The level of the aqueous phase is a very important parameter for the RLG process. On the one hand, it is to be as large as possible in order to produce a high evaporation output because it is determinative, inter alia, of the throughout efficiency of the system. On the other hand, it must not exceed a certain maximum because then the calcinate layer would break open. In that case the aqueous phase would run through the cracks in the calcinate layer and come into direct contact with the melt which may result in an interference with the normal process sequence.
In another version of the RLG process, the process vessel contains centrally arranged thermoelements which are protected by a protective pipe disposed in the center of the process vessel. The waste solution together with the glass formers is introduced into the vessel by letting the solution run down the protective pipe for the thermoelements in the form of a film, and this causes a major portion of the liquid to evaporate. The remainder of the evaporation and drying then takes place in a relatively small area around said pipe. The calcinate here forms a layer which becomes thinner radially outwardly from the protective pipe toward the wall of the vessel. This technique is intended to prevent a vary difficultly controllable process sequence which may have as a result an excess contamination of the waste gas and even clogging of the waste gas system. This mode of operation with application of the waste solution in the form of a film, is supposed to make the transition from the aqueous phase to the melt more controllable and is supposed to restrict corrosion at the vessel wall in this area. In order to keep expenditures within acceptable limits, it is considered necessary that the process be performed in stainless steel vessels. For this reason and in view of the corrosion problem, the operating temperature is generally limited to a maximum of 950.degree. C. For short periods of time it is possible to attain a temperature of 1100.degree. C. It is proposed, when using sulfate containing waste solutions, to add phosphate, aluminum, calcium, lithium or sodium ions to the waste solution during pretreatment.
A further process, called a continuous pot glass process, employs a specially designed melting crucible as the process vessel from which the finished glass melt flows via an overflow into heated storage vessels. The pretreated waste solution is fed at several points, together with the glass formers, into the cylindrical melting crucible which is horizontally disposed in a furnace. The feeder lines are water cooled in order to prevent evaporation and crust formation in the lines. In this process as well, a calcinate layer is formed from wall to wall, i.e. from the one crucible wall to a partition in the crucible arranged vertically at some distance from the outlet of the melt (overflow) and penetrating into the melt to about one half of the layer of the melt so as to prevent parts of the other layers from reaching the overflow. The throughput quantity of waste solution for this process with a crucible diameter of 500 mm and a length of 1000 mm is supposed to lie at 30 to 45 liters per hour.
In Fontenay-aux-Roses, France, a pot-glass process has been developed which is known as the Piver process. The Piver process also provides discontinuous feeding of the waste solution and of the glass formers which are mixed thereinto shortly before entrance into the process vessel. The Piver process is a discontinuous process even though the glass melt is transferred from the process vessel into storage vessels. The Piver process operates, in contrast to the above described continuous pot glass process, with a vertically disposed process vessel which, similarly to the FINGAL process or the RLG process, is inserted into a furnace which is divided into one or a plurality of heating zones. The waste solution and the glass formers are pretreated in separate systems. The glass formers are added as suspension. The waste solution and the glass former suspension are fed into the pot (process vessel), which has been uniformly preheated to about 500.degree. C., in a uniform manner in dependence on the evaporation energy until a fill level of about 75% of the total volume of the pot has been reached. During the feeding phase, evaporation takes place and the dry residue is calcinated in the lower zones of the process vessel. After the feed has been shut off, the remainder of waste solution and glass former suspension in the pot is evaporated and calcinated. Then the calcinate is melted at about 1250.degree. C. The process cycle for the pot is terminated with the discharge of the melt. Two ruthenium filters filled with iron containing granulate, a condensation and absorption system, a silica gel filter and a system for concentrating the condensate are provided to purify the waste gas. In order to remove the charged fillings of the ruthenium filters, the iron containing granulate is discharged into the process vessel where it is enclosed in the glass melt. A pilot system for the Piver process erected in Marcoule, France, employs a process vessel of 2000 mm length and about 250 mm diameter and has a throughput of waste solution of about 20 liters per hour.
In a nuclear research facility operated by the Kernforschungsanlage Julich GmbH, a process was tested which operates with a borosilicate matrix and comprises five intermediate process steps (in short FIPS). The nitric acid waste solution is passed through the following process steps in the order listed:
The process is described by M. Laser, St. Halaszovich, E. Merz and D. Thiele in Reaktortagung Dusseldorf, Mar. 20 to Apr.
2, 1976, Deutsches Atomforum e.V. (1976) pages 379-381. Denitration takes place with the addition of formaldehyde at about 90.degree. C. under a pressure of 2000 mm column of water, whereby the free nitric acid decomposes while forming nitric oxides. The denitrated and concentrated fission product solution is mixed with a slurry of the glass formers, namely, a slurry of silicic acid, borax, lime and soda. This is supposed to produce an easily pumpable suspension which is pumped by means of an immersion pump into the roller dryer. The roller dips into the suspension which causes a thin layer to adhere to it. This layer dries during rotation of the roller and is then scraped off by a blade. The result is supposed to be a well trickable powder which drops through a shaft into the melting crucible. The dry powder is melted at 1150.degree. C. to 1200.degree. C. in the same manner as in the RLG process. The nitric oxide containing waste gases from the melting crucible are freed of suspended matter and are combined with the waste gas from the denitration. This is followed by acid recovery from the nitric oxides.
All of these processes have a number of drawbacks. A grave drawback of the processes operating with a process vessel or melting crucible heated in separate zones so that three layers are formed during the course of the process, i.e. a glass melt layer at the bottom of the vessel, a calcinate layer above the glass melt layer, and a liquid or suspension layer still to be evaporated above the calcinate layer, is clearly described in German Offenlegungsschrift No. 22 45 149 in the name of Gelsenberg A.G. In such processes, for example the FINGAL process, the RLG process, the continuous pot glass process or the Piver process, there is supposed to exist the danger than larger quantities of liquid may pass through cavities or cracks in the calcinate layer and reach the hotter zones, evaporate there in an explosive manner, and carry along larger quantities of radioactive solids into the waste gas line and may even damage the melting crucible. Even without an explosive evaporation, the waste gas line is reported to clog frequently if the waste solution is introduced into the center of the crucible. To overcome this danger, the Gelsenberg process disclosed in German Offenlegungsschrift No. 22 45 149 suggests that the evaporation, calcination and melting to form phosphate glass from solutions or suspensions, respectively, of radioactive waste materials be effected along the walls of the melting crucible. The suspension is introduced into the melting vessel in such a manner so that it encounters the wall or already formed calcinate, respectively, in the upper portion of the vessel. The calcinate is disposed only at the wall of the crucible. There it is slowly melted, and drops into the phosphate glass melted disposed in the lower portion of the melting vessel. The suspension which is introduced into the melting vessel is previously concentrated in a separate vessel in the presence of hot phosphoric acid, is denitrated with formaldehyde, and then mixed with a soda solution and boiled, according to the processes disclosed in German Offenlegungsschrift Nos. 22 40 928 2nd 22 40 929. The waste gases produced during the evaporation and vitrification of the thus pretreated fed-in suspension, which gases contain ruthenium, are returned to the liquid phase present in the vessel where the concentration and denitration steps are performed.
In Great Britian, the FINGAL process developed into the HARVEST process which is supposed to permit greater throughputs and does without the two ruthenium filters. In experiments according to the HARVEST process which have thus far been performed only with simulated fission products, it has been found that if the introduction conditions of the phosphate glass process of Gelsenberg are transferred to the HARVEST process which operates with borosilicate glass so that there is an introduction of the suspension along the process vessel walls, it is possible to reduce the carrying along of certain species of the simulated fission products with the waste gas from 2.5 percent by weight by 0.1 percent. See, J. B. Morris, B. E. Chidley: International Symposium on the Management of Radioactive Wastes from the Nuclear Fuel Cycle, Vienna, Mar. 22-26, 1976 (Paper IAEA/SM/207/22).
In addition to the above, the known processes have further significant drawbacks which include the relatively small throughput of waste solution in the processes operating with discontinuous introduction, such as, for example, the FINGAL process, the RLG process or the Piver process, and the resulting high operating time per unit volume of the solidification product. Further, high expenditures are required for systems to perform the process, particularly that part of the process which occurs before introduction of the solutions into the respective process vessels, e.g. for pretreatment in the FINGAL, RLG, continuous pot glass and Piver processes, and for possible denitration of the waste solutions in the FIPS process and Gelsenberg process. Moreover, the apparatus is complex and expensive, including the high temperature furnaces which are divided into a plurality of separate heating zones with the associated relatively complicated heating programs. High costs further result from the fact that the relatively expensive process vessels are used as so-called lost storage vessels. Finally, the pumps which convey the suspensions into the process vessels or melting crucibles, respectively, are susceptible to malfunction.